Onset of slab mantle melting in Earth’s lower mantle: Evidence from ferropericlase in superdeep diamonds

Ferropericlase ([Mgx,Fe1-x] O), the most common inclusion in sublithospheric diamonds, has a poorly understood crystallization history and depth of origin. Nineteen microscopic ferropericlase grains with different Mg#s were released from Juína and Kankan diamonds with mantle-like carbon, for Mg and Fe isotopic analysis. Two groups of ferropericlase inclusions can be distinguished with respect to diamond growth: high-Mg# inclusions with mantle-like Mg and Fe (δ26Mg = −0.23 ± 0.22‰; δ26Fe = 0.00 ± 0.14‰) inferred to be preexisting and lower Mg# inclusions with non–mantle-like heavy Fe (δ26Fe up to +0.3‰) and light Mg (δ26Mg down to −1.4‰) inferred to be coeval. We propose that coeval ferropericlase inclusions formed by melting of hydrated and carbonated peridotitic slab components subducted to lower mantle depths. Continuous reaction of these melts with surrounding reduced, dry slab harzburgite can produce the large range in Mg# and Ni contents of our ferropericlase suite— a heretofore unexplained feature of global ferropericlase data.